1. Technical Field
The invention relates generally to roller cone drill bits for drilling earth formations, and more specifically to roller cone drill bit designs.
2. Background Art
Roller cone rock bits and fixed cutter bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a roller cone drill bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a well bore 14. Connected to the end of the drill string 12 is a roller cone-type drill bit 20, shown in further detail in FIG. 2.
Referring to FIG. 2, roller cone drill bits 20 typically comprise a bit body 22 having an externally threaded connection at one end 24, and a plurality of roller cones 26 (usually three as shown) attached at the other end of the bit body 22. The cones 26 are able to rotate with respect to the bit body 22. Disposed on each of the cones 26 of the bit 20 is a plurality of cutting elements 28 typically arranged in rows about the surface of each cone 26. The cutting elements 28 may be tungsten carbide inserts, superhard inserts such as polycrystalline diamond compacts, or milled steel teeth with or without hardface coating.
The cutting elements 28 on a cone 26 may include primary cutting elements, gage cutting elements, and ridge cutting elements. Primary cutting elements are the cutting elements arranged on the surface of the cone such that they contact the bottomhole surface as the bit is rotated to cut through the formation. Gage cutting elements are the cutting elements arranged on the surface of the cone to scrape the side wall of the hole to maintain a desired diameter of the hole as the formation is drilled. Ridge cutting elements are miniature cutting elements typically located between primary cutting elements to cut formation ridges that may pass between the primary cutting elements to protect the cones and minimize wear on the cones due to contact with the formation.
Significant expense is involved in the design and manufacture of drill bits to produce bits which have increased drilling efficiency and longevity. For more simple bit designs, such as those for fixed cutter bits, models have been developed and used to design and analyze bit configurations which exhibit balanced forces on the individual cutting elements of the bit during drilling. Fixed cutter bits designed using these models have been shown to provide faster penetration and long life.
Roller cone bits are more complex than fixed cutter bits, in that the cutting surfaces of the bit are disposed on roller cones, wherein each roller cone independently rotates relative to the rotation of the bit body about an axis oblique to the axis of the bit body. Because the cones rotate independently of each other, the rotational speed of each cone of the bit is likely different from the rotation speed of the other cones. The rotation speed for each cone of a bit can be determined from the rotational speed of the bit and the effective radius of the xe2x80x9cdrive rowxe2x80x9d of the cone. The effective radius of the drive row is generally related to the radial extent of the cutting elements that extend axially the farthest from the axis of rotation of the cone, these cutting elements generally being located on a so-called xe2x80x9cdrive rowxe2x80x9d. Adding to the complexity of roller cone bit designs, the cutting elements disposed on the cones of the roller cone bit deform the earth formation by a combination of compressive fracturing and shearing. Additionally, most modern roller cone bit designs have cutting elements arranged on each cone so that cutting elements on adjacent cones intermesh between the adjacent cones, as shown for example in FIG. 3A and further detailed in U.S. Pat. No. 5,372,210 to Harrell. Intermeshing cutting elements on roller cone bits is desired to permit high insert protrusion to achieve competitive rates of penetration while preserving the longevity of the bit. However, intermeshing cutting elements on roller cone bits constrains cutting element layout on the bit, thereby, further complicating the designing of roller cone drill bits.
Because of the complexity of roller cone bit designs, accurate models of roller cone bits have not been widely developed or used to design roller cone bits. Instead, roller cone bits have largely been developed through trial and error. For example, if cutting elements on one cone of a prior art bit are shown to wear down faster that the cutting elements on another cone of the bit, a new bit design might be developed by simply adding more cutting elements to the cone that is known to wear faster in hopes of reducing the wear of each cutting element on that cone. Trial and error methods for designing roller cone bits have led to roller cone bits which have an imbalanced distribution of force on the bit. This is especially true for roller cone bits having cutting elements arranged to intermesh between adjacent cones.
One example of a prior art bit considered effective in the drilling wells is shown in FIGS. 3A-3D. This drill bit comprises a bit body 100 and three roller cones 110 attached to the bit body 100, such that each roller cone 110 is able to rotate with respect to the bit body 100 about an axis oblique to the rotational axis of the bit body 100. Disposed on each of the cones 110 is a plurality of cutting elements for cutting into an earth formation. As shown in FIGS. 4, 5 and 7, the cutting elements include primary cutting elements 112 and gage cutting elements 156. The cutting elements 112, 156 are arranged about the surface of each cone 110 in generally circular, concentric rows substantially perpendicular to the axis of rotation of the respective cone as illustrated for primary cutting elements 112 in FIG. 3C. In FIG. 3A, the profile of each row of cutting elements on each cone are shown in relation to each other to show the intermeshing of the cutting elements between adjacent cones. In this example, the cutting elements comprise milled steel teeth with hardface coating applied thereon. This type of drill bit is commonly referred to as a xe2x80x9cmilled toothxe2x80x9d bit.
As is typical for modern milled tooth roller cone bits, the teeth of the bit are arranged in three rows 114a, 114b, and 114c on the first cone 114, two rows 116a and 116b on the second cone 116, and two rows 118a and 118b on the third cone 118. As shown in FIG. 3A, at least one row of teeth on each cone is arranged to intermesh with a row of teeth on an adjacent cone. The first row 114a of the first cone 114 is located at the apex of the cone and is typically referred to as the spearpoint of the bit.
The drilling performance of this prior art bit was simulated and analyzed using a method described in a patent application filed in the United States on Mar. 13, 2000, entitled xe2x80x9cMethod for Simulating the Drilling of Roller Cone Drill Bits and its Application to Roller Cone Drill Bit Design and Performancexe2x80x9d and assigned to the assignee of this invention. From this analysis, it was shown that this prior art bit has normalized cone rotation ratios for the first 114, second 116 and third 118 cones of 1.003:1.077:1, respectively, wherein the second cone 116 was found to rotate approximately 8% faster than the third cone 118. Other prior art bit designs with cutting elements intermeshing between the cones were found to have rotation ratios with differed by more than 8%. From the analysis of the bit shown in FIGS. 3A-3D, it was also observed that the scraping distance of the gage inserts 156 on each cone during drilling significantly differed between the cones. For some prior art bits, the differences between the scraping distance of the gage inserts on each cone is due, in part, to the differences between the rotation speeds of the cones.
The invention is directed to a roller cone drill bit for drilling an earth formation. In one aspect, the drill bit includes a bit body, and a plurality of roller cones attached to the bit body and able to rotate with respect to the bit body. The bit further includes a plurality of cutting elements arranged on each of the roller cones so that cutting elements on adjacent cones intermesh between the adjacent cones. The cutting elements are arranged so that a rotation speed of each cone differs by less than about 7% from the rotation speed of each of the other cones during drilling.
In another aspect, the drill bit includes a bit body, and a plurality of roller cones attached to the bit body and able to rotate with respect to the bit body. The bit further includes a plurality of cutting elements arranged on each of the roller cones so that cutting elements on adjacent cones intermesh between the adjacent cones. At least one of the cutting elements on each cone is a gage cutting element. The gage cutting elements on each cone are arranged so that the scraping distance of the gage cutting elements on each cone is substantially the same as the scraping distance of the gage cutting elements on each of the other cones.